Photovoltaic (PV) power circuits usually operate at high DC- (direct current) voltages and high currents and are thus prone to the development of electric arcs. Electric arcs can for example occur when a power line with a high current load is interrupted during maintenance or in case contacts at interconnectors are degraded. Other possible courses for arc faults, i.e. the occurrence of an electric arc in the power circuit, are corroded solder joints or broken insulators of the power lines. Arc faults are the most common fire courses in photovoltaic systems. This also reflects in the requirements for arc fault protection for photovoltaic systems as for example regulated by the national Electric Code (NEC) 690.11 of the United States of America coming into force in 2011.
A reliable arc fault detection method and system is therefore of major importance. On the one hand, for security reasons the existence of an arc fault has to be detected with a reliability as high as possible. On the other hand, the probability of an erroneous indication of a supposed arc fault has to be as low as possible, in particular if an erroneous detection of an arc fault might lead to a shutdown of a photovoltaic system without the option to automatically restart it, as for example specified in the before mentioned NEC 690.11 code.
Electric arcs usually emit a broadband AC (Alternating Current)-signal in an RF (Radio Frequency)-frequency range. Detection systems for arc faults are based on detecting an according radio frequency signal in the power circuit and are well established and for example known from document WO 95/25374. A disadvantage of such detection systems is that measurement electronics capable of performing AC- or even RF-measurements is needed.
The development or the presence of an arc does also change the DC-characteristics of a power circuit. DC-measurement equipment is usually already present in a photovoltaic power circuit, e.g. for setting a working point of a PV-generator. In order to minimize the number and complexity that have to be added to a PV-system in order to implement an arc detection system, a detection method and system based on DC-measurements within a power circuit would be desirable. However, the impact of an arc on the DC-characteristics is relatively small and accordingly, arc detections based on DC-measurements within a power circuit within a PV-power system are not known to be particularly reliable.
Document US 2011/0019444 A1 describes an arc detection system based on DC-measurements, in which a higher reliability is obtained by analyzing a DC-current signal as well as a DC-voltage signal. The presence of an arc is signaled only if it is indicated by both signals. Furthermore, after a successful detection of an arc, a distinction between a parallel and a series arc is performed by analyzing the current signal while a short cut of the generator is established.
It is desirable to create a more robust and reliable method and system for detecting arc faults in a power circuit based on DC-measurements. It is furthermore desirable to describe a photovoltaic system with a corresponding detection system.